A vat-mixer having a single mixing shaft carrying a partitioned helicoid band and blade holders with blades disposed above that band is known. (British Pat. No. 888,041). The helically-shaped band is required to transport the material to be mixed from the center of the container to its front and rear sides, while the blades are charged with returning that material from those front and rear sides to the partitioned part of the helically-shaped band in the center of the mixing shaft.
The helicoid band-segments and the blades are therefore disposed at respective inclined positions toward each other in respective halves of the shaft. It has been assumed that four mixing zones will be formed within the mixing container which will be traversed in opposite directions by the material to be mixed, and that the resulting mixing streams will be mutually overlapping as a result of the different blade attitudes as viewed by respective outgoing and incoming mixing streams at the point of their reversal.
In practice it has, however, been shown that the expected separation of the mixing stream into four separate streams which were to have flowed pairwise via the two halves of the shaft in opposite directions and which were to have overlapped at the center of the shaft, did not take place to the desired extent. The material to be mixed has rather tended to dam up at the center of the shaft at the point of partition of the helicoid band and a uniform exchange of material from one side to the other did not take place at a sufficient rate.
Another vat-mixer having a single shaft is known having an inner helix attached to one set of carrier arms and outer blades attached to another set of carrier arms. The inner helix is formed to convey the material to be mixed from one side of the container to the opposite side thereof, and the outer blades are helically-shaped and oppositely disposed to each other to convey the material to be mixed in the opposite direction. (German Auslegeschrift 1,459,258).
In a mixer of this type, therefore, only two mixing streams exist, and an exchange of material takes place more quickly than in a four-zone mixer. The experience with a mixer of the aforesaid type and its simplified construction are reasons why the two-zone mixer has proved itself in the technique of preparing concrete.
Only as the centralization of concrete preparation has progressed as a result of increased use of transport concrete and a concurrently necessary enlargement of the mixer, which frequently required a storage capability of 3000 liters in central concrete preparation-plants, has it been shown that even the proven two-zone mixer has technical, economic and size limitations. Mixers of this storage capability can no longer produce precisely guided and to all intents and purposes superimposed mixing streams, as is the case with mixers having a storage capability of up to 1500 liters. Larger two-zone mixers are additionally required to operate at too great a power, since the loaded mixer should be capable of being brought up to speed quickly. In practice the power needed for such an operation is almost never required, but the drive must be designed for that power, and this in turn has meant an increase in both manufacturing and operating costs.
The introduction of new concrete types requiring light admixtures for the production of prefabricated parts caused additional problems for mixing techniques. Here it is essential that a large amount of conventional admixtures, such as sand and gravel, be uniformly mixed with foamed plastic or expanded clay in a way as not to damage any sensitive particles of the light admixtures. Known mixers for conventional heavy cement are mostly unsuitable for such a task.